Why and how the supernatural is kept secret varies from work to work. Just from /tg/ related stuff:
- In Vampire: The Masquerade (from which the TVTropes trope gets its name) and Vampire: The Requiem, you're fucking vampires, social parasites on humanity. You don't want your prey to start hunting you.
- In Mage: The Awakening, the Masquerade is supernaturally enforced; any mundane (called "Sleeper") who sees magic causes magic to go awry, causing the Abyss to break through the magic's Supernal connections while erasing the memories of the witnessing Sleeper.
- The Skaven had something like this going on in the original Warhammer Fantasy setting. They tried to keep their existence a secret from humans; while the Empire knew about the Skaven, and didn't like them, they also recognized that breaking the Masquerade in question would lead to open warfare and a unified Skaven faction (the latter is bad since the #1 killer of Skaven is other Skaven), something they could ill-afford while fighting Chaos, beastmen, and vampires.
- In In Nomine, since divine or infernal interference can cause humans to not meet their Destiny or Fate, and Heaven is charged with trying to maximize the former and minimize the latter, and souls are the chief source of Essence for Hell, both sides have good reason to keep things on the low down.
- In Monster Hunter International the world governments believe that the more people who know about the supernatural the more powerful it becomes, and supernatural forces are always evil in this setting with many being potential world enders. As such the Monster Control Bureau and its international counterparts are often forced to intimidate, frame or even kill innocent witnesses to keep it up. The MCB director outright states in the second book that despite their best efforts he feels the truth will be exposed any day now in the wake of increasingly blatant supernatural events and modern communications.
- Urban Arcana is unusual in that the veil making shadowkind and supernatural acts look mundane is a force of nature rather than being an artificial construct. The more exposed someone is to unusual events, the weaker the effect becomes on them.
- Masque of the Red Death doesn't have one overall except for the RPGA version. As a secret society Society of the White Rose members are expected to conceal or destroy evidence of the heroic qabal's existence and avoid letting non-members in on it. Authors seem to vary on if the supernatural in general is to be hidden, though the PCs saving multiple world leaders from various supernatural events (Including, at one point, saving Queen Victoria, King Oscar II, and Kaiser Wilhelm II simultaneously) without the supernatural becoming common knowledge suggests it is intentionally hidden. If it is kept secret, it would be because magic in this setting is corrupted and will lead to further evil without care.
Non-/tg/ media, that are unusual in their approach to the idea:
- SCP Foundation universe: There are multiple organizations fighting to keep the existence of the supernatural, or the anomalous as they prefer to call it, from becoming general knowledge. One reason for this is because there are several supernatural phenomena that become more dangerous the more people that know about them. But a much bigger reason is because the world is full of extremely horrible things that the world would sleep better not knowing about, such as the girl who will give birth to something that will destroy the earth if she isn't tortured regularly. And more people knowing about anomalies means more people would abuse or try to weaponize them, with potentially catastrophic results. The Foundation itself even has an internal Masquerade because most members of the organization don't believe that magic is real even though they regularly deal with enemies that clearly have magical abilities, and those who are aware that magic is real and that anybody can learn it want to keep it a secret because they want to make sure that the Foundation doesn't become too dependent on using magic to protect the world.
- Buffy the Vampire Slayer's take was influential, in that it inspired the name and basic concept of "Sunnydale Syndrome": People being somewhat blind to the supernatural goings-on due to outside factors.
- The Dresden files (and hence, it's RPG) basically had an unwritten understanding between the various supernatural powers that they would keep their mutual existence a secret to vanilla mortals, and held up largely in part by the mortals themselves. This was largely to avoid trouble with the mortals' tendency to deny them wherever they can, and destroy them on sight where they can't to preserve their safe, sane, secular world (billions of mortals, combined with modern weapons like firearms, make burning the witch or vampire or etc. seems quaint when they can just shoot the monster). The eponymous wizard/P.I. openly advertised himself as a wizard, to the chagrin of wizard community, who accepted him grudgingly as an asset, and to the mortals, who included a few people who were aware of the supernatural powers and many more skeptics. The handful of vanilla's who saw what was going on behind the scenes and didn't get pulled in deeper, had a bad habit of rationalizing away the supernatural where they couldn't ignore it outright (and many cases where they could).
- As a side note, as of the latest book, Peace Talks, that Masquerade has either gone up in smoke, or has got everybody in the Supernatural community fighting for it, what with the leader of the enemy faction EMPing Chicago (resulting in 1,500 automobile accidents, and who knows how many deaths) and declares their intent to burn the Masquerade to the ground. It should be noted that this is regarded by everyone else as a bad idea, (including actual Fallen Angels from Hell and Vampires), as mortals actually pack a lot of punch, even without any kind of supernatural power, and murdering that many mortals in a way that shouts "terrorism" is going to provoke a reaction from them.
There are some obvious problems with most Masquerades. If you're curious, go look up TVTropes (or better yet, its uncensored fork allthetropes) article on the matter; they list quite a few usual flaws.